The present invention relates to servovalves and control systems used in connection with them. As described below, the system is particularly applicable to hydraulic systems; however, those skilled in the art will recognize that the principles applied below are also applicable systems employing compressible fluids.
Many control systems employ hydraulic circuits and hydraulic motors for control of the object system. The power supply in such systems is normally in the form of some type of pump, but the application of power is controlled by the servovalve, which controls the rate, including direction, of hydraulic-fluid flow. The servovalve is thus the interface between the hydraulic system and the previous link in the control chain, whether the previous link is electrical, mechanical, fluid, or of some other type. As in other areas of control systems, the recent availability of digital logic devices, their ease of application, and their repeatability provide impetus for the control-system designer to employ digital technology in as much of his system as possible.
An example of attempts to apply digital technology to control systems involving servovalves is U.S. Pat. No. 3,621,762 to Yo Ikebe. Yo Ikebe teaches the use of a stepper motor to position the operating member of a servovalve, pulses to the stepper motor being produced by a variable-frequency oscillator that is controlled by a reversible counter. In this arrangement, control signals that are digital in form can be applied to the reversible counter. The command information to the system is thus in digital form. This Yo Ikebe arrangement illustrates a step that has been taken toward making the servovalve system digital, but it also illustrates that servovalve control systems have heretofore been hybrid at best; none can be considered to have been totally digital.
Some of the drawbacks present in control systems heretofore used with servovalves inhere in the servovalves themselves. Ideally, a given position of the input shaft on a servovalve should consistently result in a null, or no-flow, position of the operating member of the valve. Experience has shown that this ideal is never reached, and it is only approximated with some difficulty and expense. Due to factors such as temperature changes and wear, the no-flow position of the operating member of the valve tends to vary, resulting in an offset in the control system and a consequent lack of precision. In light of this consideration, considerable expense and effort are expended during manufacturing to eliminate variation as much as possible. Even with a great deal of expense, however, variation is not completely eliminated; it is only reduced with a concomitant increase in cost.
Accordingly, it is an object of the present invention to employ in servovalve control systems the digital technology presently available to as great of degree as possible. This object is achieved by employing the teachings of the present invention, which additionally result in an arrangement that permits avoidance of much of the manufacturing expense presently incurred in eliminating the variation in no-flow position.